As described in Japanese Laid-open Utility Model Application No. 63-172241, open-type battery charging devices for supplying a charging current from a generator to a battery through a controlled rectifier circuit are widely used as devices for charging a battery by the output of a three-phase AC (alternating-current) generator driven by a motor such as an engine mounted in a vehicle or the like. The controlled rectifier circuit used is commonly a full-bridge controlled rectifier circuit configured from a hybrid bridge circuit provided with three-phase legs, in each of which either one of a top side and a bottom side comprises a thyrister and the other comprises a feedback diode. In an open-type battery charging device, the three-phase thyristors of the controlled rectifier circuit are turned on when the terminal voltage of the battery is equal to or lower than a set voltage, whereby a charging current is distributed from the generator to the battery through the controlled rectifier circuit, and when the terminal voltage of the battery exceeds the set voltage, the three-phase thyristors are turned off, and output terminals of the generator are placed in an open-circuit state, thereby stopping the supply of charging current to the battery.
As a method for controlling the on/off state of the thyristors of the controlled rectifier circuit in the open-type battery charging device, a trigger signal is applied to the thyristors of the three phases U, V, and W simultaneously when the terminal voltage of the battery is equal to or lower than the set voltage, and the trigger signals applied to the three-phase thyristors are simultaneously eliminated when the terminal voltage of the battery exceeds the set voltage, as described in Japanese Laid-open Utility Model Application No. 63-172241.
In the open-type battery charging device, when the trigger signal is applied to the three-phase thyristors simultaneously when the voltage of the battery becomes equal to or lower than the set voltage, each thyristor turns on when a forward voltage is applied across the anode and cathode thereof, and a charging current is supplied to the battery. As illustrated at the left end of FIG. 9, immediately after the charging current begins to flow to the battery, a transient state occurs in which the peak values of the positive and negative half waves of three-phase alternating-current output currents Iu, Iv, and Iw of the generator are unbalanced, but the transient state subsides as a time t elapses.
When the trigger signals applied to the three-phase thyristors are simultaneously eliminated when the voltage of the battery exceeds the set voltage, a thyristor that is off at the time the trigger signal is eliminated is no longer re-triggered, but a thyristor that is on at the time the trigger signal is eliminated is held in the on state and turned off once the anode current thereof is less than a holding current. The current of a phase in which the thyristor thereof is not re-triggered becomes zero at the time the negative half-wave current flowing through the feedback diode thereof becomes zero. For example, as illustrated in FIG. 10, when the trigger signals applied to the three-phase thyristors simultaneously are eliminated at a timing ta in a period in which the U-phase current Iu is a positive half wave, although the V-phase and W-phase thyristors are not re-triggered, the U-phase thyristor is held in the on state, and the positive half wave of the U-phase current Iu therefore continues to flow. The V-phase current Iv and the W-phase current Iw are extinguished at the time that the negative half-wave currents flowing through the V-phase and W-phase feedback diodes connected in series to the V-phase thyristor and W-phase thyristor, respectively, become zero.
The waveform of the positive half wave of the current Iu flowing through the U-phase thyristor after supplying of the trigger signal to the three-phase thyristors is stopped switches from a three-phase alternating-current waveform to a single-phase alternating-current waveform at the time that the negative half wave of the V-phase current Iv flowing through the V-phase feedback diode becomes zero. The single-phase alternating-current waveform current Iu continues to flow until the negative half wave of the current Iw flowing through the W-phase feedback diode becomes zero. If the waveform of the current Iu were to remain as a three-phase alternating-current waveform, the current Iu would vary along the trajectory indicated by a dashed line in FIG. 10 and become zero before the negative half wave of the W-phase current Iw becomes zero. However, the current Iu actually continues to flow after switching to the single-phase alternating-current waveform until the negative half wave of the current Iw flowing through the W-phase feedback diode becomes zero. Therefore, relative to a case in which the waveform of the current Iu remains as a three-phase alternating-current waveform, the timing at which the current Iu becomes zero is delayed by a certain delay time td, and the commutation margin time (time until a forward voltage is again applied to a thyristor after the current flowing through the thyristor becomes zero) of the thyristor thereof is reduced by an amount commensurate with the delay time td. In order to turn off a thyristor that is on when supplying of the trigger signal to the three-phase thyristors is stopped, and to stop the supply of charging current to the battery, the commutation margin time of the thyristor must be greater than the turn-off time (time needed for the thyristor to change from on to off) thereof.
As described above, when a thyristor (the U-phase thyristor in the example described above) that is on when the three-phase trigger signals are extinguished is turned off, the commutation margin time thereof decreases, but when a transient state in which the peak values of the positive and negative half waves of the three-phase alternating-current output current are unbalanced is not occurring in the generator, the thyristor can be turned off without hindrance. FIG. 11 illustrates the waveforms of the three-phase currents Iu, Iv, Iw in a case in which the U-phase thyristor, which is on when supplying of the trigger signal is stopped, is successfully turned off after supplying of the trigger signal to the three-phase thyristors is stopped at the timing ta. When it is detected that the battery voltage exceeds the set voltage, and supplying of the trigger signal to the three-phase thyristors is stopped, in a case in which a thyristor that was on can ultimately be turned off without hindrance, as illustrated in FIG. 11, the supply of charging current to the battery is then stopped until the terminal voltage of the battery is again equal to or lower than the set voltage, and the battery does not become overcharged.
However, when the battery voltage is detected to exceed the set voltage in a state in which the transient state of the generator has not subsided after supplying of the charging current to the battery is started, and supplying of the trigger signal to the three-phase thyristors is stopped, after supplying of the trigger signal is stopped at timing ta, as illustrated in FIG. 12, the thyristor that is to be finally turned off (the U-phase thyristor in this example) can no longer be turned off, the charging current continues to flow to the battery even after supplying of the trigger signal is stopped, and the battery may become overcharged. Such a state is prone to occur particularly when the generator is rotating at high speed.
In order to prevent such problems, a battery charging device has been proposed in which unbalancing of the three-phase output currents that occurs at the start of battery charging is suppressed by controlling the phase in which thyristors are triggered, as described in Japanese Laid-open Patent Application No. 2007-60857. According to this battery charging device, it is possible to suppress unbalancing of the three-phase output currents of the generator at the start of battery charging, and it is therefore possible to prevent a failure to turn off the thyristors when the voltage of the battery exceeds the set voltage immediately after the start of battery charging, and to prevent overcharging of the battery.